Generally, as backlights for liquid crystal displays, there are included two types called an edge light type backlight and a direct type backlight. The direct type backlights that are inexpensive and can achieve high luminance have been often used for large-sized display devices.
The direct type backlight is generally conventionally designed based on a linear light source such as a cold cathode tube, and a method for surface emitting by using a diffuser or an optical film has been employed.
However, in recent years, a shift to an LED as a light source has been required instead of the cold cathode tube from viewpoints of environmental problems, a life of light source, power saving, and picture quality enhancement.
However, since the LED is a point light source while the cold cathode tube is a line source, there is a problem that luminance unevenness becomes large. Therefore, a technology is needed for a diffuser and an optical film that can convert a point light source into a surface light source.
As an LED light source, an LED that is the most inexpensive and has Lambertian distribution in which a beam intensity of light right above the LED is high is generally often used. Therefore, it is a large problem how a light source with a strong directivity for right above the LED is converted into the surface light source.
Meanwhile, in recent years, reduction in thickness and reduction in cost have been strongly required for a liquid crystal display, and as for a backlight, a technology has been required for reducing light sources, reducing optical films, and diffusing light at a short distance from the light sources to a diffuser.
Conventionally, as a light diffusion technology for direct type point light sources, image formation positioning means have been proposed. When four point light sources are selected among a plurality of point light sources such that the four point light sources constitute a convex quadrilateral that has the smallest area and the shortest circumference length, the image formation positioning means have images of the four point light sources observed within a specific region of a light emitting surface of a light diffuser. In addition, as a shape of a light diffuser, a concave quadrangular pyramid shape having different inclined angles has been proposed (For example, refer to Patent Literature 1).
In addition, similarly, with a view to improve a utilization efficiency of light beams, a structure has been proposed in which concave quadrangular pyramids are arranged in an oblique line at a light outgoing surface side of a light diffuser (For example, refer to Patent Literature 2).
Further, as a technique for reducing light amount unevenness of an LED light source, a prism sheet has been proposed in which a plurality of prisms having corner cube shapes are formed without space at an outgoing surface side of a light diffuser (For example, refer to Patent Literature 3).